1. Field of the Invention
The present invention relates to a hydraulic jack for use in lifting a heavy-weight structure, such as a wing of an airplane.
2. Description of the Prior Art
FIG. 11 of the accompanying drawings schematically shows a known hydraulic jack for lifting a relatively large heavy-weight structure, such as a wing of an airplane.
The jack comprises a cylinder 1, a ram 3 vertically movably inserted in the cylinder 1 via a piston 2, a plurality of legs 5 each connected to an upper portion of the cylinder 1 via a bracket 4, and a caster 6 connected to a lower portion of each of the legs 5 via a spring 7 normally urging the leg 5 upwardly.
To lift a heavy-weight structure, the jack is transported to a work position under a point of the structure to be supported, and the individual casters 6 and then moved little by little manually or by a lever until the ram 3 is vertically aligned with the point of the structure to be supported.
In the event that the surface of the ground on which the jack is to stand is slanted or irregular, the individual legs 5 are expanded and shrunk to finely adjust the length of the respective leg 5 until the top of the ram 3 reaches the undersurface of the structure.
After having thus made horizontal and vertical adjustments of the cylinder 2 and the ram 3, the ram 3 is expanded to raise the structure. To lower the structure, the ram 3 is shrunk to such an extent that the load of the structure does not exert on the ram 3. At that time the leg 5 and the cylinder 2 are slightly raised by the bias of the springs 7. Finally, the individual legs 5 are shrunk manually.
However, this prior jack is disadvantageous in that since the jack is transported horizontally to the work position manually or by a lever, positioning is laborous and time-consuming. Another disadvantage of the prior jack is that the individual legs 5 must be manually adjusted in length, which is laborious and time-consuming. The larger the jack is, the more labor power will be required which would result in a poor degree of working efficiency.
FIG. 12 shows another prior hydraulic jack. This jack comprises a cylinder 1', a ram 3' vertically movably inserted in the cylinder 1' via a piston 2', and a lock nut 5' threadedly mounted on a screw portion 4' of the ram 3' outside the cylinder 1'.
When the ram 3' is expanded, compressed oil is introduced into a lower pressure chamber 5' of the piston 2' via a port 6' to raise the ram 3' and the lock nut 5'.
To keep the height of the ram 3' at a predetermined level, the lock nut 5' is lowered from the phantom-line position to the solid-line position along the screw portion 4' so that the lower surface of the lock nut 5' contacts the upper surface of the cylinder head 1'a to lock the ram 3'.
To lower the ram 3', the lock nut 5' is rotated to move to the phantom-line position, and the compressed oil is extracted from the pressure chamber 7'. As a result, the ram 3' is lowered by gravity.
This prior jack also is disadvantageous in that since the ram is locked by rotating the lock nut threadedly mounted on the ram, it is necessary to rotate the lock nut every time the ram is to be raised or lowered, thus causing a poor degree of working efficiency. Further, since the lock nut is rotated by hand, it is dangerous to position the ram.